Although the present invention can be applied generally in the field of image matching, the invention has particular utility in the area of personal identification. Matching in the context of the identification or verification of a person's identity is a burgeoning industry. While most often associated with security in sensitive government and commercial installations, matching for personal identification has potential application wherever a person's identity needs to be identified or verified, e.g., in the control of access to physical locations such as nuclear power stations, airports and other transportation facilities, industrial locations and in the home; in the control of access to computing and data management equipment; and in banking or commercial transactions. In recent years, special emphasis has been placed on developing what has come to be known as biometric identification.
Biometrics is the study of biological phenomena, and in the area of personal identification some chosen characteristic of a person is used to identify or verify that person's identity. Biometric identification has gained interest in recent years because certain personal characteristics have been found to be substantially unique to each person and difficult to reproduce by an imposter. Further, the recording and analysis of biometric data is generally susceptible to automation owing to the increased use of computer controlled electronics and digital recording techniques.
The biometric identifying characteristic may be biologically determined as with a fingerprint, or it may be some characteristic that is learned or acquired, such as handwriting or voice patterns. Ideally, the characteristic should be unique for every person and unvarying over the time frame during which the person may be tested for identification. The characteristic should also be difficult to duplicate by an imposter in order to secure against erroneous identification.
Some of the biometric characteristics most investigated today for use in a personal identification system include fingerprints, hand or palm prints, retina scans, signatures and voice patterns. Hand or palm print techniques typically evaluate the shape of a person's hand or other significant features such as creases in the palm, but these techniques may be fooled by templates or models of the hand of an authorized person. Retina scanning techniques evaluate the pattern of blood vessels in a person's retina. A drawback of this technique is that the blood vessel pattern may vary over time, e.g., when alcohol is in the blood stream or during irregular use of glasses or contact lenses. Also, a user may feel uneasy about having his or her eye illuminated for retina scanning or the possibility of eye contamination if there is contact between the eye and the scanning apparatus. Signatures can be forged easily and must usually be evaluated by a human operator, although work has been done on automated systems that evaluate the dynamics of a person's handwriting, such as the speed and the force of hand movement, pauses in writing, etc. Using voice patterns as the identifying characteristic encounters difficulties owing to the wide variations in a person's voice over time, the presence of background noise during an evaluation and the potential for an imposter to fool the system with a recording of the voice of an authorized person.
The most commonly used biometric characteristic and the one that has been the most investigated and developed is, of course, the fingerprint. Up until now, the technology of personal identification through fingerprint analysis has been used mainly in law enforcement, and this long term experience with fingerprint analysis has developed a large amount of information about fingerprints and has confirmed the uniqueness of a person's fingerprints. Historically, in law enforcement, fingerprints have been recorded by inking the fingerprint and making a print on a card for storage. Particularly for applications outside law enforcement, less burdensome and intrusive recording methods needed to be developed.
Over the past several years, there have been developed various electro-mechanical systems for recording and matching a live fingerprint with a stored representation of the fingerprint of the authorized person. In one type of system, an image of the live fingerprint pattern is read and optically compared with a master fingerprint pattern that may be stored on film. Difficulties arise in this system in aligning the live and the master fingerprint patterns, leading to the use of complicated devices to secure the user's finger in exact alignment with the recording device or to rotate and translate the live pattern with respect to the stored pattern to find registration. Further, because this type of system relies on a precise one-to-one sizing of the live and stored fingerprint patterns, errors in matching can occur where the live fingerprint pattern is deformed even slightly, e.g. when the finger is swollen or is pressed hard against the reading surface.
In another type of fingerprint matching system, the live fingerprint is read and the image is compared with a hologram of the fingerprint of the authorized person. This system requires the storage of a library of holograms at the testing location for each authorized person in the system and the use of a specialized light source and a complicated optics system.
The trend in automatic fingerprint matching is toward the increased use of electronics and computer control of the matching process, while minimizing the reliance on moving mechanical parts and complicated optics systems. In such a system the live fingerprint typically is scanned and digitally recorded as a binary, i.e., two-tone, image of the fingerprint pattern. Characteristic features of the fingerprint pattern, such as ridge endings, points of ridge bifurcation, and the core of a whorl----collectively called the minutiae of the fingerprint pattern----are found in the binary fingerprint image and then compared with master minutiae that have been derived previously from a fingerprint of the authorized person in order to determine whether there is a match.
The minutiae of both the live and the master fingerprints are defined as the X and Y coordinates of the minutiae relative to the coordinate system of the image from which they were derived. Additional data describing the fingerprint pattern can be used to reduce the chance of making a false match. These can include the "angle" of a minutia, i.e., the direction of a ridge or ridge pattern immediately adjacent to a minutia, the number of other minutiae appearing in a predetermined region surrounding a minutia point, the number of ridge lines appearing between two minutiae, or some other topological feature of the fingerprint. A significant disadvantage of this type of system is that the user's finger typically is required to be in exact alignment with the image recording device so that the coordinate system of the binary image derived from the live fingerprint and of the live minutiae is in the same orientation and position as the coordinate system on which the master minutiae are based.
The present invention presents a novel apparatus and method for matching characteristic points of a live or test image to characteristic points of a stored or master image and has particular application in fingerprint matching systems to improve upon the above described conventional systems.